Soy Products And Breast Cancer
Chester J. Zelasko, Ph.D. | November 10, 2005

One of the most common concerns we get from regular Newsletter readers is about the use of soy products after being treated for breast cancer. Women are told by their doctors to avoid all soy products, and because soy protein is used in many food bars and shakes, they’re trying to find alternatives. Is there really something to worry about? This Newsletter will examine the research behind the soy recommendation and more importantly, evaluate whether the concern is warranted.

For many years, the prevailing thought about soy intake and breast cancer was that soy was beneficial in reducing a woman’s risk of developing breast cancer. Researchers pointed to Asian populations who consume more soy products than Westernized women and have lower rates of breast cancer.

The components of soy that are thought to be beneficial are called isoflavones; the reason is that the isoflavones in soy, primarily genistein and daidzein, have a weak affinity for estrogen receptors. While the biochemistry of action is complex, it’s sufficient to say that the ability of the isoflavones to occupy estrogen receptors was thought to be beneficial in preventing breast cancer. As is often the case in nutrition, nothing is quite as simple as it seems.

Soy Research on Mice
Researchers at the University of Illinois published two studies in 2001 that raised the issue of soy products for post-menopausal women (1-2). The researchers used an animal model to represent a post-menopausal woman with breast cancer. They used mice that were born without thymus glands (useful in research because they do not reject cells transplanted from mice or humans), removed their ovaries, and then injected live human breast cancer cells. The mice were then fed a diet with and without varying amounts of isoflavones. The results were that the tumors grew larger as the amounts of dietary isoflavones increased. This led the researchers to conclude that isoflavones may be problematic for post-menopausal women who have estrogen-dependent breast cancers. The reason is that the amount of isoflavones in the diet used in the study could easily be obtained (adjusted for body weight) by women who eat soy products. These studies received a lot of attention from both the scientific community and the lay press as a result.

These studies raise several questions:
  • First, do these studies on genetically and surgically altered mice reflect what might actually happen in post-menopausal women? That’s a difficult question to answer. For example, post-menopausal women with normal-functioning thymus glands may not be affected at all by eating soy products. The permutation of possible variables is extensive and would take years to research.

  • Could soy be beneficial before menopause and not after? Some researchers believe that could be the case, but it would require further research. It may mean that women who start eating soy early in life would get benefits, but those who start eating soy after menopause would not. That’s unlikely but possible.

  • How many isoflavones are there in soy products and how much is considered moderate? Soy protein typically has 1 mg/g, soy milk has about 1 mg/fluid ounce, and tofu 7 mg/ ounce (4). From the Japanese studies, the total amount of isoflavones eaten in that culture from soy products is 30-40 mg/day. It would be reasonable to recommend that 30-40 mg/day is a moderate intake of isoflavones from all sources as part of a high-fiber, low-fat diet--also characteristic of the Japanese diet.
Should You Eat Soy?
Is soy intake something to be concerned about or not? Messina and Loprinizi published a comprehensive review of the literature in an attempt to answer the question: should breast cancer survivors eat soy products? (3) After reviewing 288 papers on soy, they concluded:

There is no scientific reason why women, with or without breast cancer, should avoid soy products based on the research to date--most of which shows soy to be beneficial.

The American Institute for Cancer Research, the American Cancer Society, and the M. D. Anderson Cancer Center are a little more conservative on their recommendations and suggest that moderate amounts of dietary soy are acceptable given the research to date.

Based on the available research, it seems that eating a reasonable amount of soy and soy products is safe. Don’t obsess about getting too much and eliminating every food that contains soy from your diet--and on the other hand, don’t seek out every soy food to eat. Strike a balance and improve all aspects of your diet.

  1. Messina, MJ and Loprinizi, CL. Soy for Breast Cancer Survivors: A Critical Review of the Literature. J. Nutr. 131: 3095S–3108S, 2001.

  2. Allred, CD, et al. Soy Diets Containing Varying Amounts of Genistein Stimulate Growth of Estrogen-Dependent (MCF-7) Tumors in a Dose-Dependent Manner. [CANCER RESEARCH 61, 5045–5050, July 1, 2001.

  3. Young HJ, et al. Physiological Concentrations of Dietary Genistein Dose-Dependently Stimulate Growth of Estrogen-Dependent Human Breast Cancer (MCF-7) Tumors Implanted in Athymic Nude Mice. J. Nutr. 131: 2957–2962, 2001.

  4. USDA-Iowa State University Database on the Isoflavone Content of Foods--1999.
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